Many wireless transceivers, such as those used in mobile communications, use a frequency synthesizer. The frequency synthesizer creates an electrical waveform, typically a varying voltage, with a desired frequency, amplitude and waveform, which may be used in, for example, frequency translations. In particular a frequency synthesizer may be used to tune a transceiver, enabling it to send and receive communications on a number of selected channels, each operating at a different central frequency.
A typical frequency synthesizer used in a wireless transceiver includes a variable oscillator linked to a feedback circuit (typically a phase locked loop or PLL). The feedback signal maintains the oscillator at a desired frequency. The feedback circuit is typically provided with two inputs: a reference clock signal from a stable oscillator (for example a fixed frequency crystal oscillator) and a frequency control signal which identifies the target frequency (typically as a fraction or multiple of the stable oscillator frequency).
One example of a communications standard in which a frequency synthesizer may be used is Time Division-Long Term Evolution (TD-LTE). In this standard, a typical configuration is where one uplink (UL) carrier and one downlink (DL) carrier share the same channel. This means that both carriers are located at the same central frequency, and share the resources of the channel on a time division basis (i.e. alternating use with time). Such an arrangement, showing the frequency allocations to both uplink (UL) and downlink (DL) carriers, is shown in FIG. 1A.
To send and receive on this channel, the wireless transceiver will generally tune the frequency synthesizer (also known in such cases as a local oscillator) to the central frequency of the carriers. As shown in FIG. 1, this central frequency (fFS) is the same for both channels. In TD-LTE, a transceiver will rapidly change between uplink and downlink.
Recently it has been proposed to use multiple component carriers to improve data rates for the upload and download links. These carriers may not be paired, that is there may be a different number of uplink carriers (UL) to downlink carriers (DL). Equally, component carriers may be activated or deactivated during a session, changing the number of carriers used. For example, 1 UL may be used with 2, 3 or 4 DLs. Alternatively, 2 ULs may be used with 3 DLs. Other arrangements will be possible.
To communicate using a group of uplink or downlink component carriers, the transceiver will typically tune the frequency synthesizer to a central frequency of the group. For example, if two downlink carriers are to be used, the frequency synthesizer may be tuned to a frequency between the two carriers. If three carriers are used, the frequency synthesizer may be tuned to a central frequency of the central one of the three carriers.
In some cases, in particular when the number of uplink component carriers is different from the number of downlink component carriers, the central frequency of the uplink component carrier(s) will be different from the central frequency of the downlink component carrier(s). This will be illustrated in a simple case of one uplink carrier, and two downlink carriers. In this case the single uplink carrier is aligned with one or other of the two downlink carriers, and thus the central frequency of the uplink carrier is different from the central frequency of the two downlink carriers.
As illustrated in FIG. 1B, it may be possible to maintain the frequency synthesizer at the same frequency for both uplink and downlink component carriers. However, this leads to an image of the reflection of the uplink carrier appearing, as shown in FIG. 1B. This is detrimental to performance, and may result in the transceiver failing regulatory standards.
An alternative option, as illustrated in FIG. 1C, is to use a different central frequency for the uplink and downlink carriers. This may be done by using two frequency synthesizers, one for the uplink central frequency, the other for downlink. However this is a complex solution and has a higher power consumption than a solution with a single frequency synthesizer. Moreover, the two frequency synthesizers may ‘pull’ each other; that is, they may converge in frequency unless carefully designed.
An alternative to using two frequency synthesizers is to use a single frequency synthesizer which is able to change frequency so as to be aligned with the central frequency of both the uplink and downlink carriers. While this prevents an image being created, it requires the frequency synthesizer to change frequencies at a fast rate.
When a frequency synthesizer changes frequencies (caused by the frequency control word being changed), there is a delay before the output frequency of the frequency synthesizer settles on the new frequency. This delay is known as the locking time. Typically, the locking time of a frequency synthesizer used in mobile communications is in the region of 100-150 μs. However, to be effective in a TD-LTE arrangement with two downlink carriers as described above, the switching time is required to be significantly less than this, such as in the region of 20 μs for example.
It is an object of embodiments of the invention to address this need.